As discussed in the incorporated documents, the use of nitric oxide (NO) to treat mammals has been known in the art for some time. The administration of nitric oxide to treat mammals generally requires that the gas be mixed with another gas, such as oxygen or oxygen-containing gas. This mixture requires very careful monitoring to be sure that the amount of nitric oxide in the gas administered to the mammal does not exceed predetermined limits. As used herein, the term “therapeutic gas” is intended to be limited to gases such as nitric oxide (NO) which are intended to treat a patient by modifying an underlying disorder in the physiology of the patient. Thus, as used herein, the term “therapeutic gas” is intended to exclude gases that do not have as their primary purpose the actual treatment of such an underlying disorder of the patient. Accordingly, gases such as anaesthesia are not included in the gas of interest here since the primary purpose of anaesthesia is not to treat an underlying disorder of the patient, but is only used as a tool to assist other means in treating a patient. Analgesics also fall into the category of gases excluded from the definition of therapeutic gas as used herein because analgesics are used for pain relief and thus treat only a symptom of a problem rather than the problem itself as is the case with the therapeutic gases such as NO and the like that are included in the definition of therapeutic gas as used herein.
Heretofore, such monitoring has been carried out using computers, or computer-based elements. Such elements are used to keep flow ratios between the nitric oxide and the mixing gas at preselected levels. However, such elements often are complicated and incorporate numerous calculations and measurements to determine the correct amount of gas to inject in order to provide a required concentration gas supply. Such systems thus have a time delay when a flow of one of the fluids changes. Often, such elements are not efficient in either high or low flow ranges or concentrations.
Still further, the complicated systems often result in loosely coupled, essentially open loop, control techniques that result in less accurate delivery over wide dynamic ranges of flow or rapid changes in flow due to the lack of feedback control.
Therefore, there is a need for a simple and accurate device and method to deliver NO or another therapeutic gas to a mammal.
There is still further need for a simple and accurate device and method to deliver NO or another therapeutic gas to a mammal through an external breathing circuit.
The need for accuracy requires a device and method for delivering such gases at a constant flow concentration regardless of inspiratory rates in order to be most effective in the treatment of diseases and injuries.
Some systems, such as the system disclosed in U.S. Pat. No. 4,932,401, use a system that set a ratio of one gas to another during the administration of gas to a patient. While this may be somewhat effective for the administration of an anaesthetic gas, such a control system may be difficult to accurately and rapidly control on the time scale of a single breath. Still further, the actual amount of one particular gas may be what is of interest and such amount may not be easily controlled if it can only be controlled as a part of a ratio. Such systems may be very inaccurate at very low rates of flow of one of the gases.
Still further, many presently-available systems must be very complicated in order to operate over a spectrum of flow ranges and concentrations. Such systems may become ungainly if all distorting factors are corrected for.
Therefore, there is a need for a device and method which has a wide dynamic range in order to deliver low concentrations into low flows and high concentrations into high flows.
There is a further need for a device and method which has a wide dynamic range in order to deliver low concentrations into low flows and high concentrations into high flows, yet is not complicated.
There is further need for a device and method which achieves accuracy associated with complicated computer-based systems yet without the attendant complications of such systems.
Some prior art systems, such as the system disclosed in U.S. Pat. No. 2,915,056, control the amount of anaesthesia gas applied to a patient according to the amount of that anaesthesia gas in the gas being exhaled by the patient. While this may be an effective means for controlling the administration of gases such as anaesthetic gases which are present in the gas exhaled by a patient, such means and methods will not be effective for the gas of interest to this disclosure which may be erratically and substantially absorbed by the patient, and thus may not be present in a deterministic ratio in the gas being exhaled by the patient. Thus, testing the exhaled gas for the presence of the administered gas will not work for systems that apply gases intended to treat the patient that are modified by the physiology of the patient.
Therefore, there is a need for a means and a method for accurately and efficiently controlling the administration of a gas that has as its primary purpose the treatment of a patient by modifying the underlying disorder in the physiology of the patient and which gas will be absorbed into the tissue of the patient.